This invention relates to a method of making a high strength duplex stainless steel and a product of this alloy in either cast or wrought form. The material of this invention displays superior toughness, weldability and cracking resistance in H.sub.2 S bearing environments compared to other duplex stainless steels of similar strength level.
In recent years, a considerable number of high strength austenitic/ferritic duplex stainless steels have been introduced, and the range of applications for these materials has expanded rapidly. The primary reason for this is that these alloys, as a class, offer an attractive combination of strength and corrosion resistance. Typically, these alloys exhibit yield strengths which are about twice those of "ordinary" stainless steels (when compared in the solution treated condition). In terms of general corrosion resistance, these alloys perform quite well in a wide variety of environments. They also have good resistance to localized corrosion and stress corrosion cracking in the presence of chlorides. In resisting these forms of corrosion, the performance of duplex stainless steels often rivals that of far more expensive, more highly-alloyed materials.
The high strength duplex stainless steels of the prior art, however, have had a number of drawbacks. Cast grades generally exhibited only moderate impact toughness at room temperature, and suffered marked losses in toughness as temperatures decreased. Duplex grades were also susceptible to serious embrittlement in the heat affected zones (HAZs) of welds. They also exhibited poor resistance to cracking in the sour (H.sub.2 S-bearing) environments often encountered in oil industry applications. These deficiencies have been major factors inhibiting even wider application of these materials.
Most high strength duplex stainless steels are designed to have a microstructure consisting of about 50% ferrite and 50% austenite. It is this microstructure which is responsible for the high strength and good corrosion resistance of these materials. In the duplex stainless steels of the prior art, the desired ferrite:austenite ratio was obtained only by controlling the composition. This prevented alloy designers from using other techniques for improving the toughness of the ferrite phase which would lead to improved toughness of the total alloy.
The current invention involves the realization that the ferrite-austenite ratio can be adjusted not only by varying the composition, but also by varying the solution treatment temperature.
By using this concept, it is possible to produce a high strength duplex stainless steel having excellent mechanical properties in both cast and wrought forms.
According to the invention a duplex stainless steel having the following composition
______________________________________ Carbon 0.001 to 0.08 Wt. % Manganese 0.001 to 2.00 Wt. % Silicon 0.001 to 1.50 Wt. % Chromium 20.00 to 27.50 Wt. % Nickel 8.00 to 11.00 Wt. % Molybdenum 3.00 to 4.50 Wt. % Sulfur 0.0001 to 0.050 Wt. % Phosphorus 0.0001 to 0.050 Wt. % Nitrogen 0.10 to 0.30 Wt. % Iron Balance ______________________________________
is produced. The composition is balanced such that: ##EQU1## where: Creq.ident.1.5(% Cr+% Si+% Mo)
Nieq.ident.% Ni+0.3(% Mn)+% Cu+22(% C)+5% N PA1 Cr=chromium equivalent.ident.1.5(% Cr+% Si+% Mo) PA1 Ni=nickel equivalent.ident.% Ni+0.3(% Mn)+% Cu+22(% C)+5(% N)
Products of this material are then solution treated by heating to a temperature in the range of 2050.degree. F. to 2350.degree. F. and then cooling rapidly as with a water quench. For cast products, the desired yield strength is developed by solution treating at a temperature selected according to the following approximate relationship: ##EQU2## where: Sy=yield strength (0.2% offset) in KSI
It should be noted that the composition ranges of U.S. Pat. No. 4,032,367 overlap those of the inventive alloy. Certain compositions of this material combined with certain solution treatment temperatures probably would give a good combination of strength and toughness. However, U.S. Pat. No. 4,032,367 does not recognize the relationships between Creq: Nieq ratio, solution treatment temperature and mechanical properties necessary to accomplish this. Obtaining a good combination of strength and toughness with the information given in U.S. Pat. No. 4,032,367 would simply be a matter of chance. Other patents such as U.S. Pat. Nos. 4,500,351 and 4,055,448 disclose preferred Creq:Nieq relationships, but they differ from those of this invention and are not directly tied to mechanical properties or heat treatment.
Compared to high strength duplex stainless steels of the prior art, the inventive material exhibits considerably greater impact toughness values, particularly at low temperatures. It also exhibits considerably greater impact toughness values in the HAZs of welds. Furthermore, the inventive material exhibits improved resistance to cracking when tested in a simulated sour gas environment according to NACE (National Association of Corrosion Engineers) Test Method TM-01-77.